An article in Le Monde (The World), a daily French newspaper, explains that the success of this book is due to a general distrust of government felt by many French due to recent corruption. It references the stereotypical, and occasionally valid, dislike of American's on behalf of the French. Even Libération, another French publication openly opposed to Bush finds the book ridiculous.
An interview with the Pentagon revealed "The Pentagon has no official reaction as we find the book too stupid to require one." (note: this was translated from English to French and back to English, and are not the Pentagon's spokesman's exact words.)
In response to Thierry Meyssan's book, Guillaume Dasquié and Jean Guisnel wrote "L'Effroyable Mensonge"(The Horrifying Lie) which denounces the conspiracy theory and blames it on the "profonde maladie sociale et politique," (the profound social and political disease) that makes the French feel they are constantly the victim of a conspiracy. Dasquié and Guisnel fear however, that their book will do nothing but bring more exposure to Meyssan's theories.
Source: Le Monde : Les Français, fous de la théorie du complot? www.lemonde.fr